Method of cleaning a filter surface in situ in a pressure filtering apparatus for liquids and pressure filtering apparatus for carrying out the method

ABSTRACT

Cleaning a filter surface (16) in situ in a filer housing (10) forming part of a pressure filtering apparatus for liquids is carried out by drying the coating deposited on the filter surface by means of hot air which is caused to flow at a moderate rate through the suspension chamber (5) of the filter housing in a direction over and substantially parallel to the sludge-coated filter surface. The dried coating is detached from the filter surface and removed from the suspension chamber by means of compressed air which alternately with the hot air and at a substantially higher rate than the latter also is caused to flow in a direction over and substantially parallel to the filter surface.

This invention relates to a method of cleaning a filter surface in situin a pressure filtering apparatus for liquids of the kind, in which thefilter surface is at least generally vertically oriented andstationarily mounted in a filter housing for dividing the interiorthereof into a suspension chamber and a filtrate chamber, between whichchambers a substantial pressure difference is maintained during eachfiltering procedure in order to separate a filtrate from a liquidsuspension supplied to the suspension chamber while leaving on thefilter surface a moist coating at least mainly consisting of finelydivided solid materials, the cleaning operation comprising the steps offirst draining off both the suspension and the filtrate from the filterhousing, subsequently drying the coating retained on the filter surfaceby supplying a gaseous drying medium, preferably air, to the suspensionchamber and eventually causing the coating to be detached from thefilter surface and to be discharged through an outlet opening located inthe lower part of the suspension chamber, the detaching of the coatingbeing promoted by supplying a gaseous cleansing medium, also preferablyair, to the filter housing during at least a period of time representingthe final stage of the cleaning operation.

The invention is also concerned with a pressure filtering apparatus forcarrying out the method, in which apparatus a fine porous filter surfacethat is at least approximately vertically oriented is stationarilymounted in a filter housing in such a manner as to divide the interiorthereof into a suspension chamber and a filtrate chamber each havingindividual outlet openings, with at least the outlet opening of thesuspension chamber being disposed at a level below the filter surface,and in which apparatus at least one inlet opening for a gaseous mediumunder pressure is provided in the upper part of the suspension chamber.

Filtering liquid suspensions on an industrial scale involves manyproblems, and for more than a century a great number of more or lesssuccessful attempts have been made to solve them. In general, all theseproblems have one and the same cause, namely the difficulty of removingthe coating, the so-called "cake", which is deposited on the filtersurface during each filtering procedure and which, when it has reached acertain, frequently rather small thickness of perhaps only a fewmillimeters, will obstruct the liquid passage up to and through thefilter surface so strongly that the capacity of the filter will becomeentirely insufficient for the existing need. In order to avoid aninterruption of the filtering process, it is common practice in such asituation to provide a filtering apparatus with at least twoalternatively operating groups of filter housings and to carry out thefiltering procedure in one of said groups while acceptable operationalconditions are being restored in the other, and vice versa.

Irrespective of whether this practice is applied or not there are onlytwo ways of restoring acceptable operational conditions in the filterhousings, namely either by replacing the clogged filter elements byclean and possibly new ones or by cleaning them in situ in theapparatus. Of these two alternatives the last-mentioned one is to bepreferred in most cases, although of course, it cannot be applied unlessthe capacity of the filters actually can be regained to a substantialdegree by such a cleaning operation.

During the last decades many different types of filter material havebeen developed, especially in the form of porous membranes, e.g. ofplastics, as well as screens of metal, which are well suited for beingcleaned in situ, particularly thanks to the fact that the filter surfaceis smooth and "fatty" or repellent so that a minimum of coating residueswill remain attached after the filter cleaning operation. The inventionis based on the use of such filter materials and especially of filtermaterials having a pore size of up to about 25 microns.

However, the possibility of cleaning the filter elements is not only amatter of the character of the filter surface but also depends on thenature of the liquid suspension or, more specifically, on the nature ofthe coating of finely divided solid materials depositing on the filtersurface. For successfully applying the method defined in theintroductory paragraph it is necessary, of course, that the coatingconsists practically solely of substances which, when being dried, willbake while shrinking into a cake that easily crumbles or disintegratesinto flakes or granules as a result of cracking. If the coating containsgreater amounts of fibrous substances which are liable to mat together,or substances which become sticky or develop strongly adhesive qualitieswhen the coating dries, such crumbling will not occur and in such a caseother, so-called wet methods for cleaning the filter surfaces must beresorted to instead.

A typical example of a previously known application of the so-called drymethod of cleaning filter surfaces in situ, which has been referred toin the introductory paragraph, is disclosed in U.S. Pat. No. 1,348,159.In accordance therewith, the drying of the filter coating isaccomplished by forcing compressed air, which is supplied to thesuspension chamber and is initially used for draining off the suspensionand the filtrate from the filter housing, through the coating and thefilter surface into the filtrate chamber so as to leave the filterhousing solely from said latter chamber. When by means of this air thecoating on the filter surface has been dried to a cracked, easilycrumbling state, the flow of the compressed air is reversed as anattempt of blowing the filter surface clean from the filtrate chamberside.

As can easily be understood, this known method has severaldisadvantages. First, it is expensive because it requires substantialquantities of air under high pressure. Secondly, it is slow becausesolely cold air is utilized and the drying effect will be low until amore extensive formation of cracks has been achieved in the coating.Thirdly, the drying will be randomly irregular because the drying airwill mainly seek its way out through the cracks first appearing in thefilter coating and, hence, will leave the portions of the coatingcontaining most moisture almost unaffected. Fourthly, the cleansingeffect obtained by backward blowing is unreliable because the cracksformed in the coating permit the compressed air to pass almost equallyeasily in both directions, and thus the purging effect of the cleansingair on the portions of the coating still adhering to the filter surfacewill be poor.

The aim of the present invention is to teach a dry method of cleaning afilter surface in situ in a pressure filtering apparatus for liquids,which will eliminate the abovementioned disadvantages of the known artand make it possible to restore acceptable operational conditions in afilter housing of the kind mentioned hereinbefore in the shortestpossible time and at reasonable cost.

According to the invention this problem is solved by carrying out thecleaning operation in the manner defined in the accompanying claim 1. Aswill be readily understood therefrom, the invention also takesadvantages of the tendency of the coating deposited on the filtersurface to shrink and crack when being dried, but--as will also appearfrom the following--according to the invention the process is carriedout under such conditions that the cleaning effect is substantiallyimproved while at the same time the need of forcing air in one or theother direction through the filter surface proper is eliminated.

According to the invention the actual drying is accomplished with only amoderate consumption of air heated to a temperature usually in thevicinity of 150° C., which air ordinarily does not need to have a higherpressure than approximately 2-3 kPa (e) in order to produce the desiredvelocity of flow of 3 to 6 meters per second and hence it can bedelivered by a fan. The drying effect is entirely independent of thepermeability of the coating and will therefore be completelycontrollable. Only the cleansing air, i.e. the medium used for promotingthe detachment of the dried flakes from the surface of the filtercoating or from the filter surface, as the case may be, has to bedelivered by a compressor, a positive air pressure of approximately 600kPa being satisfactory in most cases in order to obtain a desirablevelocity of flow of at least 30 meters per second, and even here theconsumption is moderate, because a duration of 3-8 seconds of eachcleansing period is normally sufficient and the number of cleansingperiods does not have to exceed five other than in exceptional cases.

The cleansing medium may advantageously be used also for pneumaticallydischarging the dried coating fragments released from the filter surfacethrough the outlet opening of the suspension chamber, whereby the needof mechanical means for this purpose is obviated. The manner in whichthe dried coating fragments thereafter are taken care of outside of saidoutlet opening is of no significance to the invention but, if desired,an excessive quantity of cleansing medium that is not needed forachieving the filter cleaning effect may, of course, be used to conveythem further to a collecting container which may be located at a ratherremote place. However, it is to be noted that the additional consumptionof cleansing medium resulting from this type of pneumatic conveyance ofthe discharged dried fragments has nothing to do with the economy of thefilter cleaning process itself.

In pressure filtering apparatus there are frequently used filterelements whose filter surfaces at least generally take the form of apreferably cylindrical surface of revolution having an approximatelyvertical axis. In such cases the invention is advantageously applied insuch a manner that at least the drying medium--and suitably also thecleansing medium--is caused to flow over the filter surface along asubstantially helical path, whereby the media will have to flow longdistances in contact with the coating to be removed and thus will havean optimum effect in the filter housing.

In the practical application of the method according to the invention itmay be necessary in certain cases, namely when the coating on the filtersurface has a substantial initial thickness or for other reasons shows atendency of drying in a stepwise or layer-by-layer fashion from thesurface exposed to the drying medium, to let the cleansing medium flowthrough the suspension chamber during several limited periods of timealternately with the drying medium in order to thereby cause underlying,still moist coating portions to be successively exposed. However, evenin such a case the cleaning operation should always be finished off witha forced flow of cleansing medium through the suspension chamber.

The actual cleaning of the filter surface can often to advantage beginwith a washing or leaching of the coating thereon, preparatory to thedrying of the latter, for the purpose of removing therefrom salts orother components which may act as binding agents or may delay thedrying. Such washing, if resorted to, is carried out by supplying to thesuspension chamber a vapor, typically steam, which is allowed tocondensate therein to thereby leach the coating in liquified form. Thecondensate with the dissolved coating components is then drained offfrom the filter housing in an appropriate manner. If introduced under asufficiently high pressure, the steam thus supplied to the filterhousing may also be used for draining off the suspension and thefiltrate therefrom, whereby compressed air for this purpose can bedispensed with.

As has been mentioned by way of introduction, the invention is alsoconcerned with a pressure filtering apparatus for carrying out themethod. This apparatus presents the characteristic features set forth inthe accompanying claim 8, while preferred forms of the apparatus arespecified in the claims following thereafter.

For further elucidation of the invention an embodiment thereof will bemore closely described in the following, reference being made therein tothe accompanying drawings, in which

FIG. 1 is a considerably enlarged fragmentary cross sectional view of afilter housing diagrammatically illustrating a typical example of thereaction of the sludge layer when a drying operation according to theinvention is carried out,

FIG. 2 shows in a partly simplified manner an example of a pressurefiltering apparatus for liquids comprising a plurality of filterhousings, in which apparatus the invention is intended to be applied,

FIG. 3 is an enlarged side view of one of the filter housings of FIG. 2showing further in a diagrammatical manner the equipment belongingthereto for supplying the suspension and for cleaning the filter,

FIG. 4 is a sectional elevation of the upper part of the filter housingof FIG. 3,

FIG. 5 is a horizontal cross section as viewed from the line V--V inFIG. 4, and

FIG. 6 is a sectional elevation of the lower part of the filter housingof FIG. 3.

In FIG. 1 numeral 1 indicates the outer cylindrical wall of a filterhousing of the general type illustrated in FIGS. 2 and 3, in whichhousing there is concentrically mounted a filter element comprising acylindrical, fine porous filter screen 2 supported by a central core orcarrier 3, the design of which may vary within wide limits but whichalways contains communicating cavities forming together at least a partof a central filtrate chamber 4. The filter screen 2 is radially spacedfrom the wall 1 by an interspace 5 forming a part of the suspensionchamber of the filter housing. During each filtering procedure a liquidsuspension is supplied under pressure to the suspension chamber and thusto the space 5. The filter screen 2 permits the filtrate to pass tochamber 4 but retains the dispersed substances, or at least asubstantial portion thereof, on its outer side, where these substanceswill gradually form a substantially uniform coating 6, a so-calledsludge cake. As this coating 6 successively increases in thickness thecapacity of the filter decreases and finally it will become necessary toclean the filter surface, i.e. to remove the coating, typically when thethickness of the layer has reached 3 to 4 millimeters.

As has been pointed out hereinbefore, in accordance with the inventionthis cleaning is carried out by primarily causing a gaseous dryingmedium having a temperature exceeding the vaporizing temperature of thedispersion liquid to flow over and substantially parallel to the surfaceof the coated filter screen 2 through the interspace 5 as is indicatedby arrows A in FIG. 1. In order to thereby achieve an optimum dryingeffect, the radial dimension of the interspace 5, counted from thefilter screen 2 to the inside of the wall 1, should not exceed 30millimeters, and preferably it should be of the order of 8 to 15millimeters. When being dried the coating 6 will first shrink on thesurface and crack while in most cases forming a kind of cuplike orplatelike flakes as indicated at 6'. These flakes may have differentsizes and thicknesses, depending on the composition of the coating, butpractically without exception their edges will curl outwardly in such amanner that the flakes willingly permit themselves to be released andtorn away from its base--irrespective of whether the latter consists ofan underlying portion of the coating which has not yet dried all throughor of the filter screen itself--by a flow of cleansing medium rushingthrough the interspace 5 at great speed. An additional cleansing effectis achieved by the fact that flakes already carried along by the flow ofcleansing medium will mechanically impinge and tear loose flakes whichstill adhere in the path of the cleansing medium. By alternatelysupplying cleansing medium and drying medium it is possible in this wayto remove step by step even thicker coatings than those previouslymentioned from the surface of the filter screen 2.

FIG. 2 shows as an example what a pressure filtering apparatus forliquids, in which the invention is applied, can generally look like. Theapparatus comprises eight generally cylindrical filter housings 10 eachcontaining a filter element, and these housings are vertically mountedin slightly spaced side-by-side relationship. Each of the filterhousings 10 may at its top be connected through a valve arrangement,which is not illustrated in detail, alternately either to a supplyconduit 11 for heated drying air having a fairly low pressure or to aconduit 12 for compressed air having a fairly high pressure, for example600 kPa (e). At the bottom each of the filter housings 10 mayalternatively be connected through a valve arrangement, which not eitheris shown more in detail, to a common suspension supply conduit 13 and toa discharge conduit 14, respectively. Furthermore, each of the filtratechambers in the various housings 10 is connected, through individualvalves, not shown, to a common filtrate discharge conduit 15.

The manner in which the apparatus of FIG. 2 operates will appear moreclosely from the following description of operation referring to FIG. 3.However, prior thereto and with specific reference to FIGS. 4 to 6 it isto be mentioned that each filter housing 10 houses a cylindrical filterelement 16, the upper end of which is secured to a fixture 17 forming aguide vane. Above this fixture there is a vortex chamber 18 having atangential and somewhat inclined inlet opening 19 for the gaseous mediawhich have to be supplied through a conduit 20 to the interspace formingthe suspension chamber in connection with the cleaning of the filtersurface. It should be understood that the vortex chamber 18 with itstangential inlet opening 19 as well as the fixture 17 operating as aguide vane represent means for imparting to the supplied gaseous media ahelical movement in a downward direction through the filter housingaround the filter element 16 that is centrally mounted therein, and thatthe provision of one or the other of these means may be sufficient incertain cases.

The lower end of the filter element 16 is secured in a hub-like socket21 which is hollow and supported in the filter housing 10 by a number ofspoke-like arms, of which the one designated by numeral 22 is tubularand forms an outlet conduit for the cavity of the socket 21. At itslower end the filter element 16 has a tubular pin 23 forming an outletspout for the filtrate chamber 4 (see FIG. 1) in the interior of thefilter element 16, and this pin is in a liquid-sealing manner receivedin the cavity of the socket 21, from which the filtrate thus can flowout through the conduit 22. Below the socket 21 the filter housing 10tapers at 24 in the form of a funnel towards a flanged connection piece25 which is intended to connect the suspension chamber of the filterhousing to the suspension supply conduit 13 and the discharge conduit 14(in FIG. 2) through suitable valves and pipes.

As is diagrammatically illustrated in FIG. 3, the connection piece 25 isconnected to a T-pipe 26, one branch of which by means of aremote-controlled valve 27 may be opened to the suspension supplyconduit 13 through which suspension is supplied under pressure from anarbitrary source by means of a pump P. The other branch of the T-pipe 26is connected to the discharge conduit 14 through a remote-controlledvalve 28 and furthermore through a third remote-controlled valve 29 to asuspension return conduit 30. The conduit 20 at the upper end of thefilter housing 10 is in turn through an extension 31 and a valve 32primarily connected to the hot air conduit 11, which extends from a fanF through a hot air generator 33 which, for example, may be electricallyheated. Furthermore a conduit 12 for compressed air from a compressor(not shown) is connected to extension 31 through a valve 34, as is alsoan additional conduit 35 through a valve 36. Conduit 35 may serve solelyas an outlet for air from the interior of the filter housing 10, but itcan additionally serve a double purpose so as to alternatively dischargeair or permit steam having a certain positive pressure to enter.Finally, there is a further remote-controlled valve 37 between thedischarge tube 22 and the filtrate discharge conduit 15.

In carrying out the filtering procedure, the liquid suspension to bestrained is supplied through the valve 27, whereas the filtrate isdischarged through the valve 37. All the other valves are kept closed.By means of an air cushion formed in the upper portion of the filterhousing 10 the suspension is prevented from entering into the vortexchamber 18, but nevertheless the uppermost liquid level should beslightly above the upper edge of the filter surface, approximately atthe dash-and-dot line 38 in FIG. 4. When the coating 6 on the filterelement 16 has reached such a thickness, say 3 to 4 millimeters, thatthe filter capacity is no longer satisfactory, the supply of suspensionis interrupted by closing valve 27. Instead, valve 29 is opened, whilevalve 37 is still open, and at the same time valve 34 is temporarilyopened in order to drive out the remaining suspension into the returnconduit 30 and the remaining filtrate into the conduit 15 with the aidof the compressed air. As soon as this has been accomplished valve 34 isclosed again.

As an alternative, valve 36 may be opened instead of air valve 34 and,if so, in order to supply steam under pressure to the interior of thefilter housing, this steam being used primarily for driving out thesuspension and the filtrate but, in addition, for accomplishing apreparatory washing or leaching of the sludge which is left behind inthe filter housing and above all forms the coating on the filtersurface. The steam condenses in the filter housing and dissolves saltsand possibly also other components contained in the sludge, and thecondensate is discharged from the filter housing through valves 29 and37. The washing is, of course, allowed to continue for the necessaryperiod of time, whereafter the supply of steam is interrupted, i.e.valve 36 is closed.

Besides the steam-washing making possible a purification of the sludgeresidue and a recovery of soluble salts, which latter frequently may bereturned directly to the process in which the sludge is formed, it alsooffers the further advantage, when the filters included in the apparatusare made of metal, that the danger of corrosive attacks on the filtersis reduced. At the increased temperature and in the oxygen-enrichedsurroundings created by the lively turnover of air which is aconsequence of an application of the invention, many salts and otherwater-soluble substances may develop a substantial corrosivity, and byremoving them at an early stage their effects may be avoided.

When the filter housing has been drained--after the sludge being washed,as the case may be--valves 29 and 37 are closed, and instead valves 28and 32 are opened, whereby heated air, usually having a temperature ofabout 150° C., will be blown by means of the fan F through the filterhousing 10, and more specifically through the suspension chamber thereofformed by the interspace between the outside surface of the filterelement 16 and the inside surface of the filter housing 10, at amoderate velocity, preferably 3 to 6 meters per second. As soon as aftera few minutes the drying result set forth in connection with FIG. 1 isthen usually achieved, and at the same time remaining moisture will alsobe removed from all other spaces previously occupied by the suspension.Temporarily valve 32 may now be closed and valve 34 be opened for a fewseconds in order to let cleansing air flow over the filter surface at ahigh velocity, preferably at least 30 meters per second, whereafter thehot air drying is continued.

When the remaining coating on the filter element 16 has also been dried,valve 32 is closed again, whereafter valve 34 is opened in order to letthe compressed air finally clean the filter surface and simultaneouslydrive out the released coating in the form of flakes, granules or powderinto the discharge conduit 14 and possibly also transfer it to anappropriate collecting container. When this has been done, also thesupply of compressed air is interrupted, valve 28 is closed, and a newfiltering procedure may be started. At that point first of all valve 27will be opened in order to again let suspension enter the filterhousing, the valve 36 being used whenever necessary in order to let outair and thereby adapt the size of the air cushion formed in the upperpart of the filter housing. Valve 37 is also opened for discharging theobtained filtrate. The cleaning of the filter surface is thereafterrepeated whenever necessary, usually after a period of time which isdetermined empirically.

It should be understood that the mode of applying the inventiondescribed hereinbefore is only an example which in no way should beconsidered as limiting for the possibilities of making use of theadvantages offered by the invention in practice. As a matter of fact,numerous modifications are possible within the scope of the accompanyingclaims, particularly with regard to the structure of the filteringapparatus.

I claim:
 1. A method of cleaning a filter surface in situ in a pressurefiltering apparatus for liquids of the kind in which the filter surfaceis at least generally vertically oriented and stationarily mounted in afilter housing for dividing the interior thereof into a suspensionchamber and a filtrate chamber, between which chambers a substantialpressure difference is maintained during each filtering procedure inorder to separate a filtrate from a liquid suspension supplied to thesuspension chamber while leaving on the filter surface a moist coatingat least mainly consisting of finely divided solid substances, thecleaning operation comprising the steps of(A) first draining off boththe liquid suspension from the suspension chamber and the filtrate fromthe filtrate chamber, (B) subsequently passing through the suspensionchamber a gaseous drying medium which has been heated to a temperatureensuring rapid vaporization of the liquid content of the coatingretained on the filter surface while causing said heated drying mediumto flow at only moderate velocity along a path extending substantiallyparallel to the filter surface in a manner to let said drying mediumcome into superficial contact with the coating thereon in order tothereby effect successive layerwise drying of the coating, (C) promotingdetachment of dried coating portions from any moist rest of coatingstill remaining on the filter surface as well as from the filter surfaceitself by temporarily causing a gaseous cleansing medium to flow throughthe suspension chamber along substantially the same path as said dryingmedium but at a substantially higher velocity, and (D) removing at leasta major portion of detached dried coating from the suspension chamberbefore again admitting liquid suspension thereto for resumed filtering.2. The method of cleaning a filter surface as claimed in claim 1 whereinthe velocity of flow of said heated gaseous drying medium over thecoated filter surface is limited to at most 6 meters per second, whereasthe flow velocity of said gaseous cleansing medium is at least 30 metersper second.
 3. The method of cleaning a filter surface as claimed inclaim 1 for use in a pressure filtering apparatus of the kind in whichthe filter surface at least generally takes the form of a cylindricalsurface of revolution having a substantially vertical axis, wherein atleast said heated drying medium when being introduced into thesuspension chamber is imparted a whirling motion in order to flow overthe filter surface along a substantially helical path.
 4. The method ofcleaning a filter surface as claimed in claim 1 wherein said cleansingmedium is caused to flow through the suspension chamber alternately withsaid heated drying medium during several limited periods of time.
 5. Themethod of cleaning a filter surface as claimed in claim 1 wherein thedraining off of the liquid suspension from the suspension chamber iseffected by introducing a gaseous medium under pressure into saidchamber.
 6. The method of cleaning a filter surface as claimed in claim1 wherein the coating on the filter surface prior to being dried iswashed by supplying steam to the suspension chamber.
 7. The method ofcleaning a filter surface as claimed in claim 6 wherein the steam isalso used for draining the interior of the filter housing.
 8. In apressure filtering apparatus for liquids the combination of(a) a filterhousing, (b) a filter element having a fine porous filter surface, saidfilter element being mounted in said filter housing in a stationaryposition in a manner to have its filter surface substantially verticallyoriented and to divide the interior of said filter housing into asuspension chamber and a filtrate chamber, the filter surface of saidfilter element facing the suspension chamber, (c) each of saidsuspension chamber and filtrate chamber having individual outletopenings with at least the outlet opening of the suspension chamberbeing disposed at a level below the filter surface of said filterelement, (d) means for supplying under pressure a liquid suspension tosaid suspension chamber, (e) an tangential inlet opening to saidsuspension chamber disposed in the upper part thereof, (f) means foralternately supplying to said inlet opening a heated gaseous dryingmedium under a relatively lower pressure and a gaseous cleansing mediumunder a relatively higher pressure, and (g) means for causing both ofsaid media to flow through said suspension chamber along a pathextending substantially parallel and close to the filter surface of saidfilter element so as to come into superficial contact with any coatingformed on said surface.
 9. The pressure filtering apparatus as claimedin claim 8 wherein the filter surface of said filter element at leastgenerally takes the form of a cylindrical surface of revolution having asubstantially vertical axis, and wherein said means for causing thegaseous media to flow over the filter surface comprise means forimparting to said media a helical motion about the axis of the filtersurface.
 10. The pressure filtering apparatus as claimed in claim 8wherein said filter housing has a wall member extending substantiallyparallel to the filter surface of said filter element and at a distancetherefrom that is at most 30 millimeters, the gap thus formed betweenthe filter surface and said housing wall member being a part of saidsuspension chamber to be passed by said gaseous media.
 11. A method ofcleaning a filter surface in situ in a pressure filtering apparatus forliquids of the kind in which the filter surface is at least generallyvertically oriented and stationarily mounted in a filter housing fordividing the interior thereof into a suspension chamber and a filtratechamber, between which chambers a substantial pressure difference ismaintained during each filtering procedure in order to separate afiltrate from a liquid suspension supplied to the suspension chamberwhile leaving on the filter surface a moist coating at least mainlyconsisting of finely divided solid substances, the cleaning operationcomprising the steps of(A) first draining off both the liquid suspensionfrom the suspension chamber and the filtrate from the filtrate chamber,(B) subsequently passing through the suspension chamber a gaseous dryingmedium which has been heated to a temperature ensuring rapidvaporization of the liquid content of the coating retained on the filtersurface while causing said heated drying medium to flow at a velocity ofat most 6 meters per second along a path extending substantiallyparallel to the filter surface in a manner to let said drying mediumcome into superficial contact with the coating thereon in order tothereby effect successive layerwise drying of the coating, (C) promotingdetachment of dried coating portions from any moist rest of coatingstill remaining on the filter surface as well as from the filter surfaceitself by temporarily causing a gaseous cleansing medium to flow throughthe suspension chamber along substantially the same path as said dryingmedium but at a velocity of at least 30 meters per second, (D) removingat least a major portion of detached dried coating from the suspensionchamber before again admitting liquid suspension thereto for resumedfiltering, and (E) said filter surface at least generally taking theform of a cylindrical surface of revolution having a substantiallyvertical axis, wherein at least said heated drying medium when beingintroduced into the suspension chamber is imparted a whirling motion inorder to flow over the filter surface along a substantially helicalpath.